1. Field of the Invention
The present invention relates to an image forming apparatus utilizing an electrophotographic recording process such as a laser printer, a copying apparatus or a facsimile apparatus, and more particularly to an intermediate transferring unit for transferring an image on an image bearing body onto an intermediate transfer belt and further transferring the image on the intermediate transferring belt onto a transfer material, and an image forming apparatus provided with such intermediate transfer unit.
2. Related Background Art
An image forming apparatus utilizing an intermediate transferring body has conventionally been proposed in various forms exploiting an advantage of adaptability to various transfer materials, particularly as a color image forming apparatus in which plural colors are superimposed.
FIG. 10 shows an example of the image forming apparatus utilizing the intermediate transferring belt. Referring to FIG. 10, along the periphery of a photosensitive drum 101 constituting an image bearing body, there are provided charging means 102, developing means of different colors 106 (black), 107 (magenta), 108 (cyan), 109 (yellow), an intermediate transferring belt 110 and photosensitive drum cleaner 118, and each of the developing means 106 to 109 of different colors is brought into contact, when required, with the photosensitive drum 101 by unrepresented means.
The photosensitive drum 101 is uniformly charged by the charging means 102 which is applied a negative pole bias by a bias power source 103, and irradiated by exposure means 104 as information writing means with a laser beam 105 modulated with an image signal so as to form an electrostatic latent image. Then toner as a developer which is charged in the same polarity as that of the aforementioned bias power source is supplied by the developing means 106 to 109 onto the photosensitive drum 101 bearing thus formed electrostatic latent image to visualize such latent image as a toner image. Subsequently, a primary transfer roller 111 as a first transfer member is given a voltage of a polarity opposite to that of the toner by a primary transfer bias power source 112, whereby the toner image is electrostatically transferred to the intermediate transferring belt 110. The above-described process is repeated for the developing means 106 to 109 of plural colors, thereby forming a color image on the intermediate transferring belt 110. Then a secondary transfer roller 113 as a second transfer member is given a voltage of a polarity opposite to that of the toner by a secondary transfer bias power source 114 to collectively transfer the color image onto a transfer material P such as paper, and a permanent color image is obtained by fixing means 121.
Also primary transfer residual toner remaining on the photosensitive drum 101 after the primary transfer step is recovered by the photosensitive drum cleaner 118. Secondary transfer residual toner remaining on the intermediate transferring belt 110 after the secondary transfer step is recovered by a cleaner 119. The cleaner 119 is reciprocatable in a direction indicated by an arrow, and is so controlled as to be separated from the intermediate transferring belt 110 while the toner images of the different colors are in the course of the primary transfer to the intermediate transferring belt, but contacted with the intermediate transferring belt 110 after the superposed toner images of four colors are formed thereon. The intermediate transferring belt 110 is supported under a tension by a drive roller 115, an opposing roller 116 and a tension roller 117, and is driven in rotation in a direction indicated by an arrow, by the drive roller 115.
In general, the intermediate transferring belt 110 has a thickness of 150 to 200 μm and a volumic resistivity of about 1011 to 1016 Ω·cm, and is composed for example of an electroconductive material based principally on a thermoplastic resin such as polyimide resin (PI), polycarbonate resin (PC) #21, polyvinylidene fluoride resin (PVDF), polyalkylene phthalate resin, polyalkylene terephthalate resin (PC/PAT) blend, ethylene-tetrafluoroethylene copolymer (ETFE)/PC blend, an ETFE/PAT blend or a PC/PAT blend.
However, in the image forming apparatus of the above-described conventional technology, in case transferring a color image, formed by superposing toner images of plural colors on the intermediate transferring body, onto a transfer material such as paper by the second transfer member, there may result an image defect by a transfer failure.
Such drawback will be explained more specifically with reference to a schematic view shown in FIG. 11.
FIG. 11 is a magnified view of a vicinity of the secondary transfer portion and shows the relationship between the contact state of the transfer material P with the intermediate transferring belt 110, and the transferrability, wherein an arrow X indicates the conveying direction of the transfer material P and an arrow Y indicates a pressing direction of the secondary transfer roller 113.
The transfer material P shows a concave portion (recessed portion) A and a convex portion (protruding portion) B on the surface. In the protruding portion B in a secondary transfer nip N, the transfer material P and toner T are in mutual contact, so that the toner T can be transferred onto the transfer material by the pressure of the secondary transfer roller 113 applied by unrepresented pressurizing means and by an electrostatic force generated by the secondary transfer bias applied from the secondary transfer bias power source 114 to the secondary transfer roller 113.
On the other hand, in the concave portion A of the transfer material P, if the intermediate transferring belt 110 and the opposing roller 116 have a high hardness, the pressure of the secondary transfer roller 113 exerts only a limited influence to weaken the contact between the transfer material P and the toner T, so that the transfer is executed only by the electrostatic force generated by the secondary transfer bias applied from the secondary transfer bias power source 114 to the secondary transfer roller 113 and the transferrability is deteriorated.
As the transferrability becomes different corresponding to the convex and concave portions of the transfer material P as explained in the foregoing, there result image defects such as an unevenness in the density, an unevenness in the color hue particularly in case plural colors such as a secondary color or a tertiary color are superposed, or a transfer failure causing a spot-like toner dropout in the concave portion of the transfer material P.
Such phenomena becomes particularly conspicuous in the case of a multi-color superposing where the amount of the toner T becomes larger, and, in such case, the transfer becomes difficult to realize by the electrostatic force only and evidently causes image defects such as an unevenness in color or a transfer failure causing a spot-like toner dropout.
Also the image defects based on such phenomena do not appear on a coated paper with little irregularities on the surface or a smooth paper (exclusive paper for color), but are conspicuous on a paper of an inferior surface with significant surface irregularities (plain paper or rough paper).